The present invention relates to a two stage gear shifting apparatus particularly and for example for transmissions in drive systems for ships.
It is known generally to branch off power from the main engine of a ship for purposes of driving secondary users. These secondary users, however, become dependent upon the rpm the main engine happens to have and to run in any instant. In some cases, this is of no importance. For example, a water pump as secondary user, providing cooling water for the engine, may indeed pump less water when the engine runs at a lower speed. However, the situation is quite different in other cases, such as an a.c. generator which must run at a constant rpm. Devices are known which cause such a generator to be driven at a constant speed even though the speed of the prime move varies significantly. However, the equipment needed here is quite complex and expensive, particularly if the power transfer to the generator is rather high.
The latter problem should be considered in greater detail. For normal cruising speed of a ship, its Diesel engines will indeed, or can be expected, to run at a speed which is sufficiently constant for the direct operation of an a.c. generator. In other words, relatively little is involved to normally run the ship's engine at a commensurate constant speed, which is desirable from the point of view of the ship's propulsion anyway. However, when the ship maneuvers through narrow waters, e.g. in a harbor, the engine speed is often considerably reduced, e.g. by one third, for reasons of safety. Thus, one will now need a transmission which is superimposed to increase again the effective speed to be applied to the generator. In other words, there is a need for a transmission to be interposed, if the engine speed is reduced.